Many sheet-like or film-like analysis elements for colorimetric determination of a specific component in an aqueous liquid sample have been reported so far. For example, "stick type" or "chip type" materials comprising a filter-paper matrix having impregnated therein an analytical reagent or reagents are described in Japanese Patent Publication No. 14673/69, etc. Multi-layered type integral chemical analysis elements (sheets or films) for quantitative analysis of a certain component in body fluids such as blood or urine are described in Japanese patent applications (OPI) Nos. 53888/74 (U.S. Pat. Nos. 3,992,158), 137192/75 (U.S. Pat. No. 3,983,005), 40191/76 (U.S. Pat. No. 4,042,335), 3488/77 (U.S. Reissue Pat. No. 30,267), 131786/77 (U.S. Pat. No. 4,050,898) and 142584/77 (U.S. Pat. No. 4,053,381, 4,171,246 and 4,214,968), U.S. Pat. Nos. 3,992,158, 3,526,480, etc. (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".) The latter analysis sheets have a structure comprising a transparent support with one or two reagent layers superposed thereon and, further, a porous spreading layer integrally thereon so that such is not separated from each other, or have a structure composed of the above-described two layers alone. These sheet-like chemical analysis elements are often used in a form retained in a slide frame as shown in CHEMICAL WEEK, No. 1978-Aug.-23, p. 55 or U.S. Pat. No. 4,142,863. Analysis elements comprising a combination of a slide frame and a sheet-like or film-like chemical analysis element will be hereinafter referred to as "chemical analysis slides" in this specification.
In quantitative analysis using such analysis elements, a specific component in an aqueous liquid sample reacts with a reagent contained in the analysis element, and the degree of coloration or discoloration is measured using transmitted or reflected light. Many of the reactions utilize a coloration reaction with an organic reagent in water, and hence even reactions proceeding with a faster rate require one minute or more and reactions proceeding with a slower rate, for example, those using an enzyme, often require 5 to 10 minutes under incubation at 35.degree. C. to 37.degree. C. In these reactions, the presence of a sufficient amount of water is necessary until completion of the reaction. The thickness of a reagent layer of an ordinarily used sheet-like or film-like chemical analysis element is as thin as several tens of micrometers, and therefore the water-retaining ability of the reagent layer is not sufficient. Thus, evaporation of water proceeds from the surface of the chemical analysis element to an extent that accurate quantitative analysis becomes difficult due to the change in water content in the reaction system of the reagent layer during the course of incubation or, in the case where the reduction in water content is serious, the reaction stops. In addition to the evaporation of water, migration of reaction products produced in the reaction system into other layers than the reaction layer or the detection layer, such as a light-blocking layer, a spreading layer, and the like caused by migration of water results in a failure to attain accurate quantitative analysis.
For the purpose of removing these defects, Japanese Utility Model Application (OPI) Nos. 162293/79 and 162294/79 describe the presence of a water evaporation-preventing cover on a multilayered chemical analysis element. It has been confirmed that the means described therein prevents evaporation of water from the sample-applied portion and accelerates completion of the reaction between a specific component in the sample and a reagent in the analysis element, thus serving to improve analysis accuracy.
However, based on the descriptions in these patent specifications, the applied sample comes into direct contact with the water evaporation-preventing cover, and this contact might result in staining of the sample and, in the case where the reaction between a specific component in the sample and a reagent in the analysis element requires air (or oxygen), the contact in some cases provides unfavorable results. For example, in detecting glucose, cholesterol, uric acid, neutral fats, or the like utilizing an oxidase-containing reagent, supply of air into the reaction system is necessary because the step of producing H.sub.2 O.sub.2 by the action of oxidase greatly depends upon the amount of oxygen which is also present. Therefore, the approach set forth in the above-described two utility model applications is not suitable for such a reaction system.